iandol on sat 29 dec 01
I have pondered for some time about Mel's strong statement about =
cracking a hot kiln, wondering if there might be some cogent reason why =
he would be correct when my understanding of the technology leads me to =
believe this is a suspect practice.
Perhaps the resolution to this conflict lies in a statement of F Hamer =
(I have the earlier edition of the dictionary) relating to cracking a =
kiln who gives Crack 1 and concludes "... It is cracked when dunting and =
other thermal shock hazards are passed". Hamer explains these two =
phenomena in terms of silica inversions 573C(1063F) and 226C(439F) but =
does not seem to elaborate other several mechanisms which promote the =
build up and retention of thermal stress except for the inference of =
phase changes and differential in size due to unequal temperatures in =
terms of expansion and contraction.
Given the Modulus of Rupture, which seems to be coming into fashion, of =
fired glazes and clays and the coefficients of thermal expansion of each =
of these substances together with their elastic limits, aided with some =
knowledge of their Thermal Diffusivity as well as the surface and =
interior temperatures of a piece of ceramic soon after the cold air =
rushes into the kiln it should be possible to calculate the temperature =
at which a kiln may be opened with assurance that there will be no =
disasters immediately or in the future. When all of this information has =
been factored into the equations it is just a matter of flicking fingers =
across the Abacus Beads to know whether you may crack the kiln, the =
temperature at which this might be done or if it is better to refrain =
until the kiln is cold.
Since I have neither the instrumentation nor the necessary Tables of =
Values and Constants for the materials I use I will err on the side of =
caution and wait until tomorrow, curbing my need for premature =
gratification.
Wishing all of you a great time in 2002 with lots of interesting and =
exciting discoveries about clay, glaze, and your own creativity.
Ivor Lewis. Redhill, South Australia
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